The USA had come from behind to lead 2-1 on Sunday and looked to have ended Portugal's World Cup campaign in Manaus, before Cristiano Ronalo got free on the right in the fifth minute of second-half stoppage time and picked out Varela, who powered a header past Tim Howard.

An early Nani goal saw Portugal respond well to their devastating 4-0 loss to Germany last week, but second-half strikes from Jermaine Jones and captain Clint Dempsey put the USA in front and on the verge of the last-16.

But Varela's late header ensured the top two positions in Group G remain up for grabs with Germany (four points) on top, level with the USA, while Ghana and Portugal both have slim chances of advancing with one point each.

Klinsmann picked Graham Zusi to replace the injured Jozy Altidore (hamstring), with the Sporting Kansas City midfielder lining up on the left and Dempsey moving into a lone striker's role.

The match started horribly for USA with Nani opening the scoring in just the fifth minute.

Miguel Veloso's poor cross bounced in front of Geoff Cameron and the central defender sliced his clearance to the back post, where Nani took a touch and then smashed his shot high into the net with Howard stranded.

Portugal pinned the Americans back in the opening stages with Klinsmann's side creating their first decent opportunity when Fabian Johnson, who was a regular outlet pushing up from right-back, got free in the 10th minute but his shot was deflected for a corner.

Six minutes later, Portugal's injury curse seemingly struck again with Postiga getting subbed off for Eder with an apparent hamstring strain.

The substitution seemed to halt Portugal's momentum, with Michael Bradley and Dempsey each failing to convert a pair of half chances for the USA before the half-hour mark.

Portugal reminded the USA of their counter-attacking threat three minutes before half-time with Nani unleashing a scorching drive that was parried by Howard, while in the 45th minute, the American keeper made a double save to ensure his team were just 1-0 behind at the break.

Although Howard was unimpressed by his initial save onto the post from a strike from Nani, the 35-year-old did brilliantly to keep his balance and tip Eder's follow-up over the bar.

Almeida came off at half-time with another injury, with Veloso moving to left-back and William Carvalho promoted to defensive midfield.

The second half almost started exactly as the first had, with Eder acrobatically volleying over the bar after another poor clearance from Cameron, before Johnson got forward to force Beto into a save.

In the 55th minute, Bradley should have equalised when Johnson again got free on the right and cut it back, but the Toronto FC midfielder's shot was blocked on the line by Costa.

The USA notched a deserved equaliser in the 63rd minute when a corner was cleared to Jones and the Schalke midfielder smashed his shot inside the far post, while Howard made another sharp save soon after to deny Raul Meireles.

Although Portugal had a few chances to lead again, the USA took the lead with nine minutes remaining when Zusi collected a loose ball in box and crossed for Dempsey to score with his stomach, although Klinsmann's men were denied victory thanks to Varela's late heroics.

Man of the Match: T Howards (USA)

Match Commentary - as it happened

90` + 6` That's the last of the action as the referee blows his full time whistle to end what has arguably been one of the most thrilling matches of the tournament to date!

90` + 5` Assist Cristiano Ronaldo

90` + 5` Goal Silvestre Varela

90` + 5` Cristiano Ronaldo comes to the forefront when Portugal needed him most! He sends a perfect cross into the area to pick out an unmarked Varela at the far post, and the substitute keeps the hopes of a nation alive!

90` + 5` INCREDIBLE! SILVESTRE VARELA KEEPS PORTUGAL ALIVE!

90` + 4` Another miss from Ronaldo who towers up to get on the end of a cross, but can only get the faintest of touches on the ball as it rolls behind for a goal kick. Portugal are just 60 seconds away from an early exit in Brazil.

90` + 2 `Wondolowski makes a great run into the corner and wastes more precious time. Portugal frantically trying to win it back but it's not happening for them at the moment!

90` + 1` Substitution: Graham Zusi out, Omar Gonzalez in.

90`USA wasting precious time here as they bring on another defender in Omar Gonzalez. Five minutes to be added on here, but the substitution has already taken close to one of them.

90`Nani is livid with Bruno Alves! An errant pass ends with the Manchester United winger putting the ball out of play, and he has some choice words for the defender. Tempers starting to flair for these Portuguese players.

89` Ronaldo heads wide! Fantastic ball from Joao Moutinho to pick out Ronaldo in the area. It's close to a free header, but he puts it wide! It's not so often he misses from there. Time is running out for Portugal to keep their hopes alive.

87` Substitution: Clint Dempsey out, Chris Wondolowski in.

87` The stadium erupts as Clint Dempsey is taken off. He's their hero, and may very well have booked their place in the round of 16.

85` OFFSIDE! Eder takes a shot from outside the area. It takes a deflection and falls kindly to Ronalo, but he's in an offside position! He takes the shot anyways, but Tim Howard manages to get his hands to it.

83` Still time for Portugal to find an equalizer to keep their qualification hopes alive, but the Americans are full of life after scoring a second.

81` It's a wonderfully taken goal by the USA. Yedlin makes a lovely run along the right before cutting it back for Bradley. His shot is blocked into to the feet of Zusi who crosses for Dempsey. The Sounders forward makes no mistake, chesting into the empty net from close range.

80` That's a great interception by Geoff Cameron. Moutinho tries to pick out Varela on the left, but the Stoke City defender jumps up to halt the attack. Portugal come right back through Nani, but he rather selfishly tries to do it all himself, missing the target by quite a bit with his final attempt.

79` The USA are just entering time-wasting mode as they stall ahead of taking a free kick. They want to make a substitution, but opt to wait, causing quite a stir from the Portuguese fans.

77` Portugal's mantra has changed drastically with the equalizing goal. They're sending everything forward now, taking a much more direct approach in possession. It's important to note, however, that they are out of substitutions with Helder Postiga coming off early in the first half as a result of injury.

75` Yellow Card Jermaine Jones

75` Jermaine Jones picks up the first yellow card of the match as he takes down Joao Moutinho in midfield. It was a necessary tackle from the American as Portugal had a bit of a promising attack building up there.

74` The States take the stoppage to make a substitution, bringing on 20-year-old Seattle Sounders defender DeAndre Yedlin on for midfielder Alejandro Bedoya. Meanwhile, Nani gets into the area and tries to win a penalty, but some rather poor acting can't fool the Argentinian referee who waves away the claims.

72` Substitution: Alejandro Bedoya out, DeAndre Yedlin in.

72` Fantastic stuff from Fabian Johnson, who's getting it done at both ends of the pitch for Klinsmann. He goes head to head with Varela, muscling the forward off the ball to earn a goal kick.

70` STAT ATTACK: The United States have never won a World Cup match in which they conceded the first goal.

69` Substitution: Raúl Meireles out, Silvestre Varela in.

68` Portugal going for it now, as they have to. Porto winger Silvestre Varela is on to replace midfielder Raul Meireles. Four forwards on now for Paulo Bento's men. Can they grab a second?

66` You just know Portugal will rue sitting back for so long, as they now have just over 20 minutes to find a winner and realistically keep their qualifying hopes alive.

64` Goal Jermaine Jones

The States win a corner kick that falls to Jermaine Jones at the top of the area. He has a touch to get by Nani, and curls a screamer inside the far post! Beto didn't even move. He couldn't move, it was picture perfect from the Besiktas midfielder!

64` UNBELIEVABLE! JERMAINE JONES WITH AN ABSOLUTE STUNNER!

62` Yikes! There's the counter Portugal were looking for, and it's through Cristiano Ronaldo. He gets into the area but fires horribly wide at the near post.

60` Portugal need to get back in attack. They're sitting back far too much which is giving the States reason to get forward in numbers. Portugal are waiting to strike on the counter, but it hasn't come yet.

58` The States are really pressing now! Johnson sends another cross into the box that's just out of the reach of Bradley. It's headed clear, but only as far as Jermaine Jones who sees his shot blocked.

56` Still in shock is Michael Bradley, as is the rest of the stadium. The American fans are getting louder, urging their squad on with chants of "U-S-A!" But somehow, Portugal are still in front.

54` UNBELIEVABLE! HOW HAS BRADLEY NOT SCORED?! Fabian Johnson gets on the end of another pass and breaks into the area with pace. Johnson cuts it back for Bradley, with Beto out of his net, but the midfielder can't slot into the empty net with Ricardo Costa covering. It's a potential match-saving block from the Valencia man! Bradley with his head in his hands, as he can't believe his missed it!

52` Portugal back the other way and win a corner. Jones heads clear and the States break four against two on the counter. Bradley's initial pass is intercepted though, and the attack never comes to fruition.

50` Fabian Johnson has been finding plenty of space along the right. He does so again, and gets into the area. He nearly catches Beto out at the near post, forcing the keeper to punch his shot behind. The ensuing corner is wasted, however, as it sails over the head of everyone.

48` One change to talk about at half, as Andre Almeida comes off for William Carvalho. The Sporting product will slot into midfield, with Miguel Veloso moving to left-back.

46` Substitution: André Almeida out, William in.

46` Players are back out and we're underway in the second half as Portugal kick-off...

STAT ATTACK: DaMarcus Beasley is the only player from either team who played back in 2002 when these sides met in the group stage of the World Cup, a 3-2 victory for the United States.

As for the States, Michael Bradley continues to be the architect in midfield. He's finding plenty of space and causing some headaches for the Portuguese defence. Fabian Johnson has also factored well into the attack, finding acres of space along the right flank.

Cristiano Ronaldo has showed some glimpses of his brilliance in the first half, but it appears evident that he is being hampered by his injured knee. He's taken up a central position for Portugal, but his most threatening moment of the first half came when he drifted to the left, charging at the defence before setting up Nani with a glorious chance. Is he healthy enough to make those runs throughout the second half, or will he be forced to remain in the middle as the match continues?

HALF-TIME: PORTUGAL 1-0 UNITED STATES. An early surge from Paulo Bento's men, helped by a Geoff Cameron blunder, put them in front early on. The USA took over from there, getting the bulk of the chances as they really seemed to settle into the match. Portugal came on again late as the back and forth affair continued, setting the stage for what should be a thrilling final 45 minutes in the Amazon.

45` + 2 `The USA have made their intentions quite clear, getting very physical with Cristiano Ronaldo when the opportunity presents itself. Geoff Cameron is the latest player to clash with the Real Madrid forward, and the Argentinian referee decides that's enough for the first half here in Manaus.

45` Two minutes to be added on to the end of the first half...

44` NEARLY A SHOCKER FROM TIM HOWARD! Nani takes a harmless looking shot that seems to handcuff the Everton keeper. He nearly makes a mess of it, fumbling it off the post. The rebound falls for Eder who appears to have doubled Portugal's lead, but Howard makes up for his mistake with a fantastic reactionary stop to deny the Braga forward.

42` Ronaldo rushes over to take the free kick, but in the end his attempt is harmless as it sails well over the bar. Chance wasted for Portugal.

41` Lightning quick break from Portugal! Ronaldo finally gets forward with a burst of speed and picks out a lovely pass for Nani who sees his shot saved well by Howard. The danger isn't totally cleared on first try, and Nani wins a free kick in Ronaldo territory.

39` There's a bit of a stoppage here as both sets of players make their way to the touch line to grab some water. It's come at an awkward time as well, with the half rapidly approaching.

37` Ronaldo has taken up much more of a central position for Portugal. He's hardly wavered onto the wings, which could be a tactical decision to avoid long runs on the flank as a result of his injury. Regardless of why, it's certainly harmed their ability to break from the back as they love to do with the lead.

35` STAT ATTACK: Portugal have kept a clean sheet in five of their last seven World Cup group stage matches.

33` Portugal get a rare attack in, but it ends with a shot deflected into the path of Ronaldo who was standing in an offside position. They've got nothing going in the final third, with the Stars and Stripes posing much more of an attacking threat since the opening goal.

31` The States are firing from all cylinders at the moment! A giveaway in midfield allows Dempsey to break. He picks out Fabian Johnson, streaking up from the right. The defender cuts into the middle and nearly slots in at the near post with Beto pinned to the ground.

29` Another long range attempt from Michael Bradley who is finding plenty of space so far. Again it's just wide of the far post, but Portugal can't keep giving him clean looks at goal or he may just make them pay.

27` Crucial block from Ricardo Costa! Graham Zusi picks up the ball to the edge of the area, turns and picks out Dempsey with a nice pass, who takes it between two defenders before having a shot from close range. Costa slides in however to turn it aside.

25` Portugal's game plan seems rather obvious in the opening quarter of the match. They're comfortable passing amongst themselves at the back, to try and lure USA in so they can hit them on a fast break.

23` BRADLEY! The Toronto FC midfielder has a shot of his own from 25 yards out that blisters just a couple yards over the bar. The States are beginning to ask some questions of the Portuguese defence who haven't replied with much certainty as of yet.

21` Portugal appear to have gotten a bit complacent since their opener, and are showing far less attacking intent. Joao Moutinho picks up the ball in midfield with options for a pass, but opts to take a shot from 35 yards out that rolls helplessly wide of Tim Howard's goal.

19` There's a large Portuguese contingent in the crowd. Portugal are back in possession, but Cristiano Ronaldo is taken down by a clean tackle from Matt Besler. Crowd is urging the ref to call a foul but he was in perfect position and waves away the claims.

17` Fantastic ball from Bradley! Shortly after the restart, and the Americans continue to apply the pressure. Michael Bradley sends a perfectly weighted through ball to Dempsey, but his shot from a tight angle is punched away by Beto. The ensuing corner falls again for the striker, but his flick sends the ball past the crowd and wide of the far post.

16` Substitution: Hélder Postiga out, Éder in.

16` Whoa! There's a delay now as Raul Meireles and Helder Postiga are both down injured. Meireles had a coming together with Beckerman, and the American player appeared to lash out with an elbow that the referee seems to have missed. Meireles appears to be alright, but Postiga will not be able to carry on as he will have to be replaced by Eder.

14` NEARLY THERE FOR DEMPSEY! The Seattle Sounders forward has a go at goal from the free kick, but puts it just inches over the bar at the near corner. Beto was fully stretched to try and keep that out.

13` Lovely give-and-go worked by the Americans. Johnson makes his way up from right-back and takes a Jones pass straight at the heart of the Portuguese defence. Bruno Alves commits to the challenge and the States win themselves a free kick from about 30 yards out.

11` Much better from the States who finally get some time on the ball. Bedoya gets forward and behind the left-back Almeida, but his cross is blocked and put behind for a corner that they can't take advantage of.

9` STAT ATTACK: The United States have now conceded the first goal in seven of their last 10 World Cup matches.

7` And now a corner for Portugal who are on the front foot early on here! The USA have to get rid of their early jitters and get on with the match or it could prove to be a long day for them.

5` Goal Nani

Horrible mistake from Geoff Cameron, who scuffs his clearance straight into the feet of Nani who slots home comfortably from five yards out.

5` GOOOOOOOAL! It took less than five minutes, and Nani has Portugal in front!

3` Timid start from Portugal. They gain possession early on, passing it amongst themselves in defence. Ronaldo gets the ball, much to the delight of the Portuguese supporters in attendance, but sends an errant pass forward for Joao Pereira as it runs out of play for a USA throw-in.

1` The United States kick-off, moving left to right, wearing their all white kits. Paulo Bento's men are are decked out head to toe in classic Portuguese red attire.

Luckily for the players and fans alike, while it is another hot day in the Amazon, the pitch will be completely covered in shade for the duration of the match. There's a slight breeze that promises to pick up as we move into the night, which will hopefully help the quality of football in the later stages of the game.

Jurgen Klinsmann has taken more of a conservative approach to the match, putting forth a rather defensive starting XI. The German coach has opted not to replace the injured Jozy Altidore, instead starting Clint Dempsey as the lone man up top in front of a five man midfield. Who will slide up to support the former Tottenham striker? Will Michael Bradley, Graham Zusi and Alejando Bedoya pose enough of a threat going forward to expose a decimated Portuguese back four? We'll find out in just a few minutes time!

There will be added pressure on Portugal's midfield trio, who will perhaps have to over-compensate for the loss of Pepe and Coentrao. Joao Moutinho will be the all-important link in midfield, and will look to provide service to the front three that was sorely lacking against Germany.

Portugal limp into this clash without four of their regular starters. Goalkeeper Rui Patricio, defender Fabio Coentrao and forward Hugo Almeida are all forced out of the line-up due to injury, while Real Madrid centre-back serves a suspension for his headbutt on Thomas Muller in the opening match. Beto, Andre Almeida, Ricardo Costa and Helder Postiga are set to replace them, but how much will Portugal be affected by the losses? And just how fit is Cristiano Ronaldo, who is warming up with a tight wrap on his injured knee?

The United States come into the match in much different shape, having beaten Ghana in their first match thanks to a late John Brooks winner - his first international goal. Jurgen Klinsmann's men can clinch their place in the knockout stages with a win today, while a draw would even be enough to give the Stars and Stripes a foot into the round of 16.

It doesn't get much bigger than this for Portugal. Paulo Bento's men are unable to qualify for the knockout stages today, but they can be eliminated with a loss. The Seleccao barely survived a disastrous first match to Germany, suffering a 4-0 defeat, and losing four members of their starting XI in the process. A draw would keep their qualification hopes alive, but it would make for a nervy final match day, where even a win may not be enough for them to qualify.